JAXA 数値解析グループ 燃焼 乱流セクション. Jul 13, 2012

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1 噴霧の微粒化 燃焼初期過程の詳細解析および実用シミュレーションへの展望 新城淳史 JAXA 数値解析グループ 燃焼 乱流セクション Jul 13, 2012

2 Background Liquid sprays for various engineering applications Our main interest tis in liquid fuel id lengines Rocket engine Others.. Automotive engine Jet engine Spray Agriculture

3 Recent research topics Combustion instability and control (lean premixed CH 4 swirl combustor) Shift from gas fuel to liquid fuel for real engine applications Tachibana et al., PCI 2007; Shinjo et al., CNF 2007 Liquid atomization physics Spray combustion simulation We still have many issues (physical mechanisms and modeling)! Rayleigh breakup Spray combustion physics Turbulent atomization Shinjo & Umemura, IJMF 2010, 2011, PCI 2011 Group combustion Shinjo & Umemura, PCI (in press)

4 Spray simulation issues Primaryatomization atomization region The incoming boundary of spray simulation cannot be resolved easily cannot be treated in the Lagrangian way However, good models are not available now Convergent Science We>>O(1) We~O(1) We<<O(1) Physical mechanisms should be understood for modeling Droplets Ligaments 2 U l We

5 1. Liquid atomization study High speed Diesel like like fuel injection

6 Objectives and approach Investigate the primary breakup mechanisms Obtain modeling dli insight i Very detailed numerical simulation is used No breakup model is employed Navier Stokes eqs. + surfacetension Direct interface capturing by level set/vof method Surface tension by CSF method

7 Recent primary atomization research High resolution data are becoming available to understand the underlying physics Simulation Experiment Liquid jet simulation in cross flow (Left: Pai et al. 2008, right: Herrmann 2010) Exp photo of spray front (Wang et al. 2008)

8 Our simulation of Diesel like jet Overall characteristics and ligament/droplet dynamics (Shinjo & Umemura, IJMF 2010) Surface instability development and atomization (Shinjo & Umemura, IJMF 2011) Role of jet head (Shinjo & Umemura, PCI 2011)

9 Numerical methods Euler Euler two phase flow 3D Navier Stokes equations Gas and liquid are Newtonian fluids Pressure by Poisson equation Advection by CIPmethod Interface capturing by Level Set method with MARS (VOF) method Surface tension by CSF method

10 Flow conditions Liquid jet from a round nozzle Three injection velocities No excitation at the nozzle is given to exclude nozzle effects such as cavitation or liquid flow turbulence. = Atomization is initiated from jet front only Case Nozzle diameter D=2a Ambient pressure p Gas density g Liquid density l Liquid viscosity l Surface tension coefficient Liquid velocity U l =U Gas velocity U g Bulk liquid Reynolds number Bulk liquid Weber number 1 30m/s e m/s Pa s 0.1mm 3MPa 34.5kg/m 3 848kg/m e-3 3 0m/s N/m e-6 Pa s 100m/s Re bulk We bulk 2 Ua U a

11 Computational resources Liquid jet from a round nozzle (D=0.1mm) JAXA JSS system Case # of grid points Spatial resolution billion 1.5 m 2 1.2billion 0.75 m 3 6 billion 0.35 m 4 6 billion 0.35 m # of CPUs (# of cores) 320 (1280) 480 (1920) 1440 (5760) 640 (2560) Computational time (Real clock time) 750hrs 700hrs 400hrs 1100hrs One of the world s largest computations ti at this time in this field.

12 (1) Overview Ligament/droplet scale As the liquid id Wb Weber number increases, generated td structures get smaller Case 1 (30m/s) Case 2 (50m/s) Case 3 (100m/s)

13 Ligament formation from front Gray: vortices by the second invariant of velocity gradient tensor Case 1 (a) t=18.27 (b) t=19.67 (c) t=21.07 Case 3 (a) t=14.39 (b) t=15.11 (c) t=15.83 Roll up vortices (circumferential and lateral) create ligaments.

14 Ligament formation from jet core Case 1 (a) t=18.97 (b) t=19.67 (c) t=21.07 Case 3 (a) t=15.76 (b) t=16.33 (c) t=16.91 Flat crests break into multiple ligaments.

15 From spray front Ligament formation Circumferential and lt lateral lvortices create ligaments formation From jet core A hole forms on crest structure and multiple ligaments develop. Ligament condition: We g ~O(1) We u u g g l g L 2 a a L : ligament radius =(Aerodynamic force)/(surface tension)

16 Droplet formation Case 1 p p ( pg al) ( al) a L : ligament radius Tip bulb contracts and pushes the central part. Tip bulb pressure is high and wavy patterns appear on the ligament. 3.6a L (c) t=18.97 (d) t=19.67 Tip bulb grows and the pressure becomes lower. First neck is destabilized. Liquid jet emerges from the first neck to the tip bulb. Pinch off will soon follow. (e) t=20.37 (f) t=21.07 Short wave mode

17 (2) Head atomization loop From theoretical consideration, l [ Vouter( inner) Vsheet /2] V * : Edge velocity (fixed on spray front) V / sheet Uc Vsheet lasheet a sheet : Thickness at edge l: Spatial interval CFD result satisfies the above relations : Surface tension : Wavelength on edge Vˆ ˆ ( inner) 1.0, V 0.5, R: Edge radius ˆouter sheet 0.37, l 0.6, a L 2 : Recirculation zone length sheet 0.04, : Time interval of atomization ˆ 0.8

18 (3) Liquid core surface instability High speed liquid jet U=100m/s, t=8.27 2D (axi symmetric) patterns U=100m/s, t= D patterns

19 Initial 2D patterns and TS mode Initial 2D patterns Re=Recr Initial 2D patterns Unstable 1 : displacement thickness Without jet head wake effect With jet head wake effect modification K x 0 U 1 dx b x U b Ut b x Ut Re K 1 ln Ut b x x 2 b Ut 2b Ut x Ut The results suggest that the initial 2D surface patterns are generated by TS waves.

20 Vortex structures t=11.33 Streaks by velocity Similar to wall turbulence transition! *True especially for high speed liquidinjection injection t=15.47

21 Summary of study 1 High speed liquid fuel injection Atomization ti characteristics ti and mechanisms identified d in this configuration Modeling study underway Investigationof of sprays of other configurations Impingement injector, wind wave, air blast atomizer, etc. Two liquid mixing for a rocket engine injection element Wind wave simulation

22 2. Spray combustion study Vapor mixingand early flame kernel development

23 Background Droplets and group combustion Single droplet combustion Internalgroup combustion External group combustion External sheath combustion In real sprays, however, the combustion mode is not well understood. Group combustion diagram by Chiu (PCI 1982)

24 Previous work in the literature Eulerian Lagrangian simulation Turbulence scale >> droplet scale Gas phase autoignition Ignition occurs at small SDR Viggiano (CNF 2010) Wang & Rutland g (PCI 2005)

25 Previous work in the literature Simplified droplet array Flame spreading mode analysis Umemura (J. JSME 2002) Mikamiet al. (CNF2005) D il d / i ibl Detailed measurement/computation possible Gap with the real sprays (scale, turbulence effect, etc.)

26 Objectives and approach Investigate autoignition of dense spray region usingthe previous droplet atomization data Path leading to group combustion Droplet distribution and numberdensity Gas phase turbulence Droplet shape effect Find input to modify current models Detailed simulation with evaporation, mixing and reaction Flow field around each droplet is resolved Computational domain is limited to small scale *J. Shinjo, A. Umemura, Proc. Combust. Inst. (in press)

27 Gas Liquid Gas Computational setup Domain fixed on the head Small spatial and temporal scales Flame cannot be simulated, but early flame kernel development can be captured. Gas 900K ambient air from tˆ atm Equilibrium q evaporation n heptane (C 7 H 16 ) global onestep reaction (Westbrook & Dryer, CST 1981) Physical properties from NIST data 2.2billion grid points Nozzle diameter D N Ambient pressure p Air density g Liquid density l Air temperature T g Liquid temperature T l 0.1mm 3MPa 11.5kg/m 3 848kg/m 3 900K 300K Liquid injection velocity U l =U 100m/s (Lab frame) Air initial velocity U g 0m/s (Lab frame) Liquid viscosity l 2.87e-3 Pa s Air viscosity g 3.90e-5 Pa s Surface tension coefficient 30.0e-3 N/m 27

28 Numerical methods Navier Stokes equations Density, velocities, temperature t and chemical species Compressibility included (perfect gas equation of state) CIP for advection terms Level set interface capturing C bi d ith VOF t th d f l ti Combined with VOF type method for volume conservation Evaporation Equilibrium evaporation Jump condition at interface Chemical reaction One step global reaction of n heptane

29 Flow field Recirculation zone behind the head Color: droplet number density (1/m 3 ) *2D slice image makes the impression that the number of droplets is small.

30 Mixing of vapor/air Region A Z tˆ tˆ tˆ tˆ ˆ *Evaporation starts at t

31 Mixing of vapor/air Region B Z tˆ tˆ tˆ tˆ ˆ *Evaporation starts at t

32 Mixing of vapor/air Region A Z Z Vorticity tˆ tˆ tˆ Strong correlation between mixing and vorticity Faster mixing in region B Region B ˆ *Evaporation starts at t

33 Droplet/turbulence interaction Droplet/turbulence scales Kolmogorov scale 3 14 uij, uij, ~ 1 m 0.23D 32 D 32 : Sauter mean diameter Droplets (gray) and eddies (Q isosurface) near region B Slip velocity exists between droplet and gas Vorticity generation (eddy diameter~8 12 ) Global energy feed from liquid to gas Droplet Stokes number S D U L S t 2 t l g ~22

34 Vorticity around droplet Near droplet Wake region Region A (low number density) *Red scatter points are from single droplet simulation of the similar Re (~40) with evaporation and reactions. Region B (high number density) Wake interaction is stronger in region B 13 Mean inter droplet distance: (A) l=7.9d 32 and (B) l=3.3d 32 l ~ n

35 Formation of vapor clusters Region B Initially Cluster formation *Red shows the high reaction rate, but this is still not a flame Fuel consumption rate (mol/cm 3 /) /s) 35

36 Formation of vapor clusters Note that the reaction rate magnitude is small at this early stage. *Red shows the relatively high reaction rate, but this is still not a flame. *Also note that the spatial scale is much smaller here. Fuel consumption rate (mol/cm 3 /s) /) Neophytou & Mastorakos CNF 2009 Typical reaction rate magnitude 36

37 Color: reaction rate Black white: vorticity Formation of vapor clusters Molecular diffusion d 2 d L Ddif Or tˆ e6s 5.4e-6s With D dif =3.4e-6m 2 /s L=D 32 =4.3 m ˆ t Eddy turnover e e 4 ω 3.6e-7s Or t ˆ 0.36 With ω 3.5e7 s 1 Actual time difference ˆ t tˆ Already connected! Eddy mixing is dominant.

38 Group combustion number Inregion B, G nd R c With R c =0.25D N n=3.4e14 m -3 D 32 =4.3 m External Group Combustion Mixing by droplet generated vorticity Spherical and non spherical droplets Inter droplet interaction Especially E i for high h number density

39 Scale extension Based on DNS...possible by using a larger computer system such as K class system) Spatial scale Temporal scale At least, the primary atomization region should be covered. It is still far short compared to the ignition delay time. Current simulation time scale Viggiano (CNF 2010)

40 Outlook Combustor scale simulation Hybrid method DNS or Model based boundary conditions LES We will return to the initial starting point but with more knowledge of physics and modeling of spray and flame

41 Summary of study 2 Atomization data extended with evaporation, mixing and reactions Droplet/turbulence interaction is very important in mixing External group combustion expected in dense region Modeling Must include the above effect We are working on this now Others Detailed reaction mechanisms Spatial/temporal scale extension

42 Thank you!